Tertiary oil recovery process utilizing a preflush

ABSTRACT

An adjusted salinity and hardness, aqueous preflush containing a thickening material which can be, for example, a hydrophilic material such as polyacrylamide, polysaccharide, polyvinyl aromatic sulfonate, polyethylene oxide, methyl cellulose, etc. or colloidal silica, etc. in a concentration sufficient to increase the viscosity of the adjusted salinity aqueous preflush fluid is injected into a subterranean, petroleum-containing formation. Optionally, the preflush fluid may contain a sacrificial agent, such as sodium tripolyphosphate, etc. The high salinity formation water is efficiently displaced by viscous low salinity aqueous preflush fluid to increase the effectiveness of a subsequently injected high salinity sensitive single surfactant solution. The surfactant may be displaced by a mobility buffer solution, which then may be displaced by water.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for recovering petroleum fromsubterranean formations by the injection of flood water containing asurfactant or mixture of surfactants. More particularly, this inventioninvolves the injection of a preflush fluid of adjusted salinity waterhaving therein a thickening agent to efficiently displace the highsalinity formation water.

2. Prior Art

Many subterranean petroleum-containing formations contain natural energyin the form of active bottom water drive, solution gas drive, or gas capdrive, in sufficient quantity to drive the petroleum to the productionwell from which it can be transported to the surface. This phase of oilrecovery, known as primary recovery, recovers only a portion of thepetroleum originally in place. When the natural energy source has beendepleted, or in those formations where insufficient natural energy wasoriginally present to permit primary recovery, some form of supplementaltreatment is required to recover additional oil from the formation.Water flooding is by far the most economical and widely practicedsupplemental recovery procedure. Water flooding is accomplished byinjecting water into the formation via one or more injection wells. Theinjected water displaces and moves the petroleum toward one or moreproduction wells, where it is transported to the surface. Water floodingis also quite inefficient, and approximately 50 percent or more of theoriginal oil remains in the formation at the termination of conventionalwater flooding operations.

Numerous factors are responsible for the failure of water flooding torecover a high percentage of the oil remaining in the formation afterprimary recovery. A low viscosity fluid displaces a higher viscosityfluid quite inefficiently, because the low viscosity displacing fluidchannels through the high viscosity fluid. The displacement efficiencycan be related mathematically to the mobility ratio of the displacingand displaced fluids. Various additives have been proposed in the priorart to alleviate this problem. Hydrophilic polymers which increase theviscosity of the displacing fluid, improve the mobility ratio anddecrease the tendency for the displacing fluid to channel or finger intoand inefficiently displace the higher viscosity petroleum. U.S. Pat. No.3,039,529 (1962) discloses the use of polyacrylamide polymer to increasethe viscosity of injected water to improve the mobility ratio and hencethe oil displacement efficiency of an oil recovery process. U.S. Pat.No. 3,282,337 describes the use of polyethylene oxide as a thickener forinjection water for the same purpose.

The immiscibility of water and petroleum, and the high surface tensionexisting between water and petroleum also contribute significantly tothe inefficient displacement of oil by water. The use of a surfactant tolower this surface tension will improve the displacement efficiency. Forexample, U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycolether as a surfactant in an oil recovery process. U.S. Pat. No.3,032,713 (1967) discloses the use of a particular petroleum sulfonateas a surfactant for oil recovery products. U.S. Pat. No. 3,468,377describes the use of petroleum sulfonate having a specified molecularweight distribution as a surfactant for oil recovery.

The combined use of a surfactant solution to decrease the surfacetension between the injected aqueous fluid and the petroleum containedin the formation, and a solution of a polymeric material to improve themobility ratio and sweep efficiency provide a very efficient petroleumrecovery process. For example, U.S. Pat. No. 3,477,511 (1969) describesthe use of a surfactant solution followed by thickened water to displacethe surfactant solution through the formation. Many other combinationsof surfactants and water thickening polymers have been proposed, allsharing the common feature of specifying that the surfactant mustprecede the viscous fluid for optimum recovery efficiency.

Most of the chemicals which have been proposed in the above-citedreferences for improving the mobility ratio of the injected fluid andfor reducing the surface tension between the injected fluid and theformation petroleum, require a specific, low salinity fluid environmentto function effectively.

The salinity sensitivity of the most desirable surfactants for use inoil recovery has a substantial impact on the economics of a proposedsupplement recovery operation employing a surfactant. While it has beengenerally recognized in the industry for many years that surfactantscapable of reducing the interfacial tension between the injected fluidand the formation petroleum would improve the oil recovery efficiency ofa supplemental oil recovery program, it has never been demonstrated thatthe additional oil which can be recovered under field conditions issufficient to justify the cost of the surfactant. This is especiallytrue because of the enormous quantity of surfactant which must beemployed in a field, in order to have a significnt effect on thedisplacement efficiency. If high formation water salinity results in ashift in surfactant choice to a higher cost material or if a greaterconcentration of surfactant must be used, the cost of a surfactant floodwill be increased substantially. It is known, however, that manymillions of barrels of oil remain unrecovered in a petroleum reservoirat the conclusion of conventional water flooding operations, and with animpending shortage of readily recoverable crude oil, it is becoming amatter of paramount national importance to devise a reasonablyeconomical method of recovering this oil.

SUMMARY OF THE INVENTION

In the present invention, there is provided a new and improvedsurfactant flood oil recovery process whereby high salinity formationwater is effectively displaced by injecting into a petroleum-bearingformation via an injection well a preflush of a thickened lower salinityaqueous fluid selected from the group comprising (A) an aqueous fluidhaving dissolved therein a small amount of hydrophilic polymer, (B) anaqueous fluid containing a small amount of colloidal silica, and (C) anaqueous fluid having dissolved therein a small amount of hydrophilicpolymer and a small amount of colloidal silica. Useful polymers include,for example, polyacrylamide, polysaccharide, methyl cellulose,polyethylene oxide, or polyvinyl aromatic sulfonate, etc. In a secondstep an aqueous surfactant solution which may, if desired, also containa polymeric thickening agent, is injected into the formation via theinjection well and finally the petroleum displaced by the injections isrecovered through a production well.

Various surfactant systems which have been formulated to perform information waters of moderate salinity and hardness [e.g., 80,000 ppm TDC(total dissolved solids)] have been described in the patent literature.There are reservoirs which have very high salinity and hardness (e.g.,200,000 ppm TDC) connate water. Most often the reservoirs are waterflooded with fresh water, thus lowering the salinity and hardness offormation water to moderate levels. In actuality, most reservoirs(especially limestone reservoirs) are heterogeneous. Because ofpermeability heterogeneities and the poor sweep efficiency of waterflooding, formations end up with zones of varying salinity and hardness.Such formations have unfavorable salinity conditions for successfulsurfactant flooding. Surfactant systems tailored for certain salinityand hardness levels do not perform effectively at quite differentsalinity and hardness environments. Poor oil recovery is expected fromtertiary oil recovery processes applied to reservoirs with heterogeneouspermeabilities. The reason being in a tertiary oil recovery process thesurfactant slug followed by a mobility control slug because of theimproved mobility control will invade zones which were not contacted bywater during water flooding. Oil will not effectively be mobilized fromsuch zones because of the existing unfavorable salinity and hardnessconditions.

In the improved process of this invention the reservoir is preflushedwith a slug having the desired salinity and containing a mobilitycontrol agent. This preflush slug will improve the sweep efficiency andcontact areas unswept by water during the previously performedwaterfood. Thus, the wide salinity distribution within the heterogeneouspay zone will be narrowed to a range suitable for the surfactant systemdesigned for that reservoir.

Also it is desirable to maintain a favorable mobility control during thesurfactant injection by either employing a surfactant system ofviscosity matching that of the preceding preflush slug (which contains amobility control agent) or to incorporate a mobility control agent intothe surfactant slug. The process of this invention will effectivelycondition a heterogeneous reservoir and narrow the wide salinityvariations existing within a pay zone, thus resulting in higher oilrecovery efficiency than would be obtained with surfactant flood,including no such preflush.

DETAILED DESCRIPTION OF THE INVENTION Type A Fluids

Type A fluids as described above generally will have dissolved thereinfrom about 0.01 to about 0.10 percent by weight of a hydrophilic polymersuch as polyacrylamide. Such aqueous fluids exhibit a viscosity of fromabout 6 to about 15 centipoises (measured at a shear rate of 300reciprocal seconds) and are sufficient to effectively increase theefficiency of displacement of high salinity formation water by thepreflush solution.

Type B Fluids

Colloidal silica useful in this invention in Types B and C fluidsdescribed above is different from precipitated silica or silica gel. Thecolloidal silica useful in this invention is a fumed silica which ismade up of chain-like formations sintered together. These chains arebranched and have enormous external surface areas of from about 50 toabout 400 meters² /gram and each segment in the chain has many hydroxyl(OH) groups attached to silicon atoms at the surface. When the segmentscome into proximity to each other, these hydroxy groups will bond toeach other by hydrogen bonding to form a three dimensional network.

Colloidal silica acceptable for use in the method of this invention willgenerally have a particle size ranging from about 7 to 15 millimicrons(mμ). In this size range the colloidal silica will pass through evenreservoirs with very small pore size. For example, a reservoir havingvery low permeability of say 0.016 millidarcies (md) has acorrespondingly small pore size of 25 to 100 mμ. Thus, the colloidalsilica suitable for use in the process of this invention will passthrough even the smallest pores encountered in hydrocarbon reservoirsand will maintain a constant viscosity in the displacing fluid.

Colloidal silicas are readily available from a number of manufacturers.One source is the Cabot Corporation of Boston, Mass. which marketscolloidal silica under the trade name CAB-O-SIL. Colloidal silica isalso available from other commercial sources and the reference to onesource is not intended to limit the scope of this invention.

When the silica particles are dispersed in liquid medium, the networkstructure formed by the silica particles restricts the movement of themolecules of the liquid medium and results in an increase in theviscosity of the liquid.

Generally the Type B fluids will contain from about 0.05 to about 0.06percent by weight of colloidal silica and preferably they will containabout 0.05 to about 0.1 percent by weight.

If desired, the Type B fluid can contain from about 0.001 to about 0.01percent by weight or more of a surfactant based on the weight of thefluid which can be, for example, a soap, the sodium salt of a highmolecular weight sulfate or sulfonate, etc. Generally, the surfactantemployed will be of the anionic type as exemplified by surfactants ofthe formula ##STR1## where n is an integer of from 2 to about 10.

Type C Fluids

In this invention the Type C fluids as described above will contain fromabout 0.01 to about 0.10 percent by weight of the hydrophilic polymerand from about 0.05 to about 0.60 percent by weight of colloidal silica.The thickened low salinity aqueous preflush fluid may be followeddirectly by the surfactant solution utilized in this invention, thechoice of which will be influenced principally by formation andpetroleum characteristics of the particular reservoir in which thesupplemental recovery procedure is conducted. It is sometimes desirableto use an unreacted water isolation slug between the preflush solutionand the surfactant solution. Alternatively, the polymer concentration inthe preflush fluid may be decreased gradually or tapered to more nearlymatch the viscosity of the following surfactant solution. The surfactantsolution may then be displaced by the injection of water, or it may befollowed by a thickened water to more efficiently move the surfactantsolution through the formation. Depending on the existence of otherproblems which may be anticipated or known to exist, other chemicaltreating agents may be included in the preflush slug. For example,precipitating or chelating agents for removing interfering polyvalentions, sacrificial adsorption agents which adsorb on the formation rockto prevent adsorption of the subsequently injected surfactants, orchemicals to densensitize water sensitive clays present in the formationto prevent permeability loss due to swelling of such clays on contactwith fresh water, may be added to the preflush solution. By the use ofthis low salinity aqueous preflush fluid, many chemicals such asdetergents, emulsifiers, foaming agents, etc. which are sensitive tohigh salinity environments, may be used in formations having relativelyhigh salinity formation water.

Surfactants of the type described for use in the Type B fluids may alsobe utilized, if desired, in the Type C fluids. The amount of thesurfactant employed in the Type C fluid will be from about 0.001 toabout 0.01 percent by weight or more based on the weight of the fluid.

In practicing this invention, a thickened aqueous fluid of Type A, B, orC as described above is injected through an injection well and into thesubterranean petroleum-containing formation. The salinity of thisaqueous solution should be less than the salinity of the formationwater, and preferably from about zero to about 100,000 parts permillion. Frequently, this operation will be conducted in a formationwhich has previously been subjected to conventional water flooding,although this is not essential for the practice of this invention. Theaqueous preflush solution containing the polymeric thickener orcolloidal silica, etc. is followed by an aqueous surfactant solution,which is in turn followed by the injection of water. Water injection iscontinued, and the injected water displaces the oil within thesubterranean petroleum-containing formation and moves it, together withthe surfactant solution, toward one or more production wells. Energy forthe displacement process is furnished by the pumps injecting the aqueousfluids and water into the injection well, which forces the oil throughthe formation to the production wells and then to the surface of theearth.

Numerous polymers are usable in our invention, and one of the mostsatisfactory is a hydrolyzed polyacrylamide which may be graphicallyillustrated by the following formula: ##STR2## wherein Y representshydrogen, ammonium, and alkali metal or an alkaline earth metal, Rrepresents hydrogen or a metal radical, X represents chlorine, a loweralkoxy or acyloxy group or a cyanide radical, m ranges from 12 to 67, nranges from 33 to 88, p ranges from 0 to 10, and the sum of m, n, and pequals 100, and z is at least about 60. This class of polymers is knownfor the purpose of increasing the viscosity of the injected fluid andthe efficiency with which the injected fluid displaces petroleum,specifically being disclosed in U.s. Pat. No. 3,039,529 (1961).

Other excellent polymers for use in the aqueous preflush solution forthe practice of the subject invention are the polyvinyl aromaticsulfonates having relatively high molecular weights and yet beingsoluble in water. Polymers of this class have the following generalformula: ##STR3## wherein Ar is divalent monocyclic hydrocarbon unitderived from benzene or benzene derivatives, R is hydrogen or a metalion, and M is hydrogen, sodium or potassium. The use of this polymer formobility ratio improvement in oil recovery operations is more fullydetailed in U.S. Pat. No. 3,282,337 (1966).

Another class of hydrophilic polymers which can be used to thicken thelow salinity aqueous preflush of the subject invention is the flocculantgrade water soluble polyethylene oxides such as are described in U.S.Pat. No. 3,021,231. The use of such polyethylene oxides to thickeninjected water for displacement of viscous crude oil is described inU.S. Pat. No. 3,282,337 (1966).

Another class of hydrophilic polymer water thickening materials suitablefor use in this invention is the polysaccharide compounds, many of whichare readily available commercially. For example, in U.S. Pat. No.3,208,518 there is disclosed a water flooding process wherein theviscosity of the flooding medium is increased by the use of highmolecular weight polymers, specifically an ionic polysaccharide producedby the fermentation of carbohydrates by bacteria of the genusxanthomonas, under controlled pH conditions.

From about 0.02 to about 0.5 and preferably from 0.1 to about 0.3 porevolumes of the aqueous preflush solution containing the viscosityincreasing polymer or colloidal silica should be injected into theformation for the purpose of displacing the high salinity formationwater from the flow channels in advance of the surfactant flood. It isdesired that a sufficient volume of the preflush solution be used toinsure that the discreteness of the solution is maintained throughoutthe formation. The quantity of preflush solution required will depend onthe well spacing of the particular pattern being employed. The lower endof the pore volume range may safely be used in the instance of fairlyclose well spacing whereas the higher end of the range is preferredwhere large well spacing is employed since the solution will besubjected to greater radial spreading and mixing as it progressesthrough the formation in advance of a surfactant. A small volume tendsto lose its discreteness in a field using large well spacing.

If the surfactant solution to be injected after the preflush solutionhas a viscosity less than the viscosity of the preflush solution, itwill be necessary either to provide an isolation slug of from about 0.05to about 0.5 pore volumes of untreated water between the preflushsolution and the surfactant solution, or to taper the concentration ofthe viscosity increasing hydrophilic polymeric material in the preflushsolution so the preflush solution viscosity at least equal to andpreferably less than the viscosity of the subsequently injectedsurfactant solution.

A wide variety of surfactants may be employed in the practice of thisinvention. Generally, the concentration of the surfactant or surfactantsin the solution injected into the formation will be from about 0.05 toabout 20.0 percent and, preferably, from about 0.05 to about 5.0 percentby weight. The quantity of the surfactant solution injected will be fromabout 0.02 to about 0.50 pore volume.

An especially useful surfactant composition for use in this inventioncomprises a solution of a mixture of a petroleum sulfonate and asolubilizer such as a polyethoxylated alkyl aryl sulfonate or sulfateand alkali metal and ammonium salts thereof. Suitable sulfonates includethose having the formula: ##STR4## wherein R is alkyl of from 8 to 22carbon atoms, n is an integer of from 2 to about 10 and A is selectedfrom the group consisting of hydrogen, sodium and potassium andammonium. Usually the petroleum sulfonate will constitute about 65percent by weight of the surfactant mixture although this may be variedsomewhat depending on the salinity of the reservoir water, the specificsurfactants used, etc. One example of a useful surfactant mixture ofthis type comprises the following materials:

Petroleum Sulfonate: Blend of high and low equivalent weight petroleumsulfonates.

Solubilizer: ##STR5## or the sulfate.

A second group of surfactant compositions suitable for use in theprocess of this invention comprises a solution of a mixture of asolubilizer which can be, for example, a polyethoxylated alkyl arylsulfonate or sulfate of the same type as previously described and alkalimetal and ammonium salts thereof of the same type as previouslydescribed and a synthetic sulfonate, such as an alkyl benzene sulfonatewherein the alkyl group has from 8 to 22 carbon atoms as exemplified bydodecyl benzene sulfonate, hexadecyl benzene sulfonate, the potassiumsalt of octadecyl benzene sulfonate, etc. Generally, the polyethoxylatedalkyl aryl sulfonate will constitute about 40 to about 60 weight percentof this surfactant mixture and the actual concentration employed willdepend on the salinity of the reservoir water, the particularsurfactants utilized, etc. An example of a suitable surfactant mixtureof this type consists of the following compounds:

Synthetic Sulfonate: ##STR6##

Solubilizer: ##STR7## or the sulfate.

A third group of surfactant compositions suitable for use in the processof this invention comprises a solution of a polyethoxylated alkyl arylsulfonate or sulfate of the same type as previously described, apolyethoxylated alcohol sulfonate or sulfate and the alkali metal orammonium salts of these two materials. Polyethoxylated alcohols suitablefor use in preparing the polyethoxylated alcohol sulfonate or sulfatehave the formula:

    RO(CH.sub.2 CH.sub.2 O).sub.m H

wherein R is alkyl of from 8 to about 22 carbon atoms and m is aninteger of from 2 to about 18. Polyethoxylated alcohol sulfates usefulin the method of this invention have the formula:

    RO(CH.sub.2 CH.sub.2 O).sub.m SO.sub.3 A,

wherein R and m have the same meaning as described above and A isselected from the group consisting of hydrogen, sodium, potassium andthe ammonium ion, and the final sulfonate material, i.e., thepolyethoxylated alcohol sulfonates has the formula:

    RO(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 A,

wherein R, m and A have the same meaning as described above. Examples ofsurfactants useful in formulating solutions of the type described aboveinclude:

(a) Sulfonate: ##STR8##

Sulfate: ##STR9##

(b) Sulfonate: C₁₆ H₃₃ --O(CH₂ CH₂ O)₃ CH₂ CH₂ SO₃ Na or

Sulfate: C₁₆ H₃₃ --O(CH₂ CH₂ O)₄ SO₃ Na

Petroleum sulfonates which are presently among the more popular classesof surfactants being considered for supplemental oil recovery techniquesmay also be used in this process. The various materials available underthe general name of petroleum fraction used for sulfonation and in thedegree of sulfonation imparted to the petroleum fraction. A preferablepetroleum sulfonate is described in U.S. Pat. No. 3,302,713 (1967)disclosing a petroleum sulfonate prepared from a petroleum fractionwhose boiling range is from 700° F., to 1100° F., which corresponds to amolecular weight range of from about 350 to about 600. The sodium saltof the sulfonation product of this petroleum fraction is an excellentmaterial for use in the subject invention.

Petroleum sulfonates are particularly desirable surfactants to use foroil recovery operations because of their low cost per unit weight andrelatively high surface activity. One serious shortcoming, however, istheir low tolerance to salinity. For this reason, petroleum sulfonatesare especially benefitted by the subject invention, which provides ameans for displacing high salinity formation water so the surfactant canfunction more effectively and in a relatively low concentration range.The upper limit of salinity tolerance for most commonly used petroleumsulfonates is around 2 percent or 20,000 parts per million, although theoptimum performance is realized at a lower salinity. If the salinity ofthe formation water is in the range of 1 percent or less, of if it canbe reduced to a level of 1 percent or less, the petroleum sulfonatesolution will exhibit its maximum degree of surface activity and willfunction most efficiently at a lower concentration. Ordinarily, theconcentration of petroleum sulfonate in the surfactant solution shouldbe from about 0.05 percent to about 15 percent and preferably from about1.0 percent to about 5.0 by weight. If a particular viscous crude is tobe displaced, it is necessary to include in the surfactant solution aquantity of hydrophilic polymer to increase the viscosity of thesurfactant solution so that a more favorable mobility ratio will existbetween the surfactant solution and the displaced formation petroleum.It is preferably to use the same hydrophilic polymeric material in thesurfactant solution as was used in the preflush solution. In any eventthe viscosity of the surfactant solution should be slightly greater thanthe viscosity of the preflush solution, and so from about 50 to about5000 and preferably from about 200 to about 1000 parts per million ofthe viscosity increasing hydrophilic polymer should be used therein. Thesurfactant solution will tend to accumulate droplets of oil dispersed inthe solution, so the viscosity of the surfactant solution will increaseas it progress through the formation. For this reason, it is preferableto add a greater amount of the viscosity increasing hydrophilic polymerto the first portion of surfactant solution injected into the formationand lesser amount of polymer to the latter portions.

Type B and C fluids may contain in addition to the ingredients alreadymentioned a small amount of a friction reducing water-soluble polymer.These friction reducing polymers are effective in fresh water, brinesand other fluids and in addition they are temperature stable and shearstable. Finally, they are effective at los concentrations. The frictionreducing polymers employed in this invention are random, linear(including lightly crosslinked, high molecular weight water-solublepolymers containing at least 1 percent by weight of(3-acrylamido-3-methyl) butyl trimethyl ammonium chloride and/or itsanalogs.

The friction-reducing polymers useful in this invention can be preparedby free radical polymerization methods. Polymerization by such a methodmay be affected by contacting the monomer or monomers such as(3-acrylamido-3-methyl) butyl trimethyl ammonium chloride and, forexample, acrylamide or partial hydrolyzed acrylamide. Preferably, thefriction reducing copolymer employed in this invention will be acopolymer consisting essentially of from about 1.0 to about 100 percentby weight of (3-acrylamido-3-methyl) butyl trimethyl ammonium chlorideand from 0 to about 99 weight percent acrylamide partially hydrolyzedacrylic acid, methacrylic acid, methacrylamide, allyl alcohol,acrylamide, N-vinyl pyridene, N-vinyl pyrrolidone or N-vinyloxazolidone. The amount of the friction reducing polymer employed influids B or C of this invention will range from about 10 to about 1000parts per million by weight based on the total weight of the fluid.Usually the copolymer employed as friction reducing polymer will have amolecular weight of between about 100,000 and about 1 million or more.The preparation of these friction reducing copolymers is more completelydescribed in U.S. Pat. No. 3,868,328 which is incorporated herein byreference in its entirety.

Field Example

The following example of field implementation of the subject inventionis offered only for purposes of illustrations, and is not meant to berestricted or definitive of the entire invention, the scope of whichwill be defined more precisely hereinafter in the Claims.

A petroleum-containing formation located at a depth of 5600 feet isexploited by means of conventional water flooding operations using aninverted five spot pattern, until the water-oil ratio rises above 30.The formation thickness is 30 feet and the porosity is 26 percent. Inthis inverted five spot pattern the center well is employed as aninjection well while the four remaining wells serve as production wells.The dimensions of the square grid on which an inverted five spot patternis based is 500 feet and it is known that only 75 percent of thereservoir will be swept by the injected fluid using the standard fivespot pattern. The pore volume of the pattern swept by the injected fluidwill be 500 × 500 × 30 × 0.26 × 0.75 = 1,462,500 cubic feet. Thesalinity of the water contained in the formation is 225,000 parts permillion which is considerably above the tolerable salinity for petroleumsulfonate and other commonly used surfactants. A total of 0.1 porevolume (146,000 cubic feet) of a preflush solution having a salinity ofonly about 100,000 parts per million and having dissolved therein 0.03weight percent of polyacrylamide, 0.01 weight percent of a frictionreducing copolymer which is random, linear water-soluble copolymerconsisting essentially of about 1 percent by weight of(3-acrylamido-3-methyl) butyl trimethyl ammonium chloride and about 99weight percent acrylamide and having dispersed therein about 0.02 weightpercent of colloidal silica is injected into the formation via theinjection well. This is followed by the injection into the formation of0.1 pore volume (146,000 cubic feet) of aqueous solution havingdissolved therein 2.0 percent by weight of a sulfonate surfactant of theformula: ##STR10## and also having dissolved therein 0.02 weight percentpartially hydrolyzed polyacrylamide to increase the solution viscosityto about 9 centipoise. Next 0.1 pore volume (146,000 cubic feet) ofwater containing 0.03 weight percent of partially hydrolyzedpolyacrylamide and having a viscosity of 9.4 centipoise is injected intothe formation. This is followed by the injection of water having asalinity of about 100,000 parts per million to displace the oil,surfactant solution and thickened water through the formation. Oil isproduced through the associated production wells in the five spotpattern, and the amount of oil produced is substantially in excess ofthat produced by water flooding alone.

We claim:
 1. A method of recovering petroleum from a porous,subterranean, petroleum-containing formation containing saline waterwhich is penetrated by at least one injection well and at least oneproduction well which comprises:(a) injecting an aqueous preflush fluidcomprising a mixture of water, colloidal silica having a particle sizeranging from about 7 to 15 millimicrons and a random, linear,water-soluble friction-reducing polymer of a monomer or monomersconsisting essentially of from about 1 to 100 percent by weight of(3-acrylamido-3-methyl) butyl trimethyl ammonium chloride and from 0 to99 percent by weight of acrylamide or partially hydrolyzed acrylamidewherein the said aqueous preflush fluid has a salinity less than thesalinity of the formation water, into the formation, via an injectionwell, (b) injecting an aqueous surfactant solution into the formationvia the injection well, and (c) recovering petroleum displaced by thesaid injections through a production well, wherein the said aqueoussurfactant solution of step (b) comprises an aqueous solution of asurfactant composition selected from the group consisting of: (I) apolyethoxylated alkyl benzene sulfonate having the formula: ##STR11##wherein R is alkyl of from 8 to 22 carbon atoms, n is an integer of from2 to about 10 and A is selected from the group consisting of hydrogen,sodium, potassium and the ammonium ion, a polyethoxylated alcoholsulfate having the formula:

    RO(CH.sub.2 CH.sub.2 O).sub.m SO.sub.3 A,

wherein R and A have the same meaning as previously described and m isan integer of from 2 to about 18, or a polyethoxylated alcohol sulfonatehaving the formula

    RO(CH.sub.2 CH.sub.2 O).sub.m-1 CH.sub.2 CH.sub.2 SO.sub.3 A

wherein R, m and A have the same meaning as previously described (II) amixture of a polyethoxylated alkyl benzene sulfonate as described in (I)above together with an alkyl benzene sulfonate, and (III) a mixture ofpetroleum sulfonate and a polyethoxylated alkyl benzene sulfonate asdescribed in (I) above.
 2. The method of claim 1 wherein the saidaqueous preflush fluid also contains a surfactant having the formula:##STR12## wherein n is an integer of from 2 to about
 10. 3. The methodof claim 1 wherein the said water in the aqueous preflush fluid issaline water.
 4. The method of claim 1 wherein the said aqueoussurfactant solution is a saline solution.